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Old 05-31-2019, 10:15 AM   #1
tomormatt
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Add concrete to patio for exterior tile

The patio is 6 x 20 with an existing slab 6" thick and previously had bricks. The patio was raised by tree roots so it had a negative grade towards the house. My original thought was to simply tile over the existing bricks after using a self-leveling compound; however, the front of the patio was raised higher than the door frame so too high even before leveling.

I had to chip out a trench for an electrical conduit to the panel and am taking the opportunity to pop all the bricks off, level the slab, and install tile. My question is on adding concrete to the existing slab.

The bricks are approximately 2" thick and I need to allow for the tile thickness. I can chip out some of the concrete if necessary, but would really like to avoid that (chipping bricks out was PITA, but much easier than having to break the slab).

My plan is to drill holes and epoxy rebar to tie the new pour into the old slab. My concern is the thickness as the new pour would be between 1.5" - 2.0". Any issue using something like Quikrete 5000 for the new slab? Planning on allowing 28 days to cure before I tile unless anyone thinks 14+ days is sufficient.

I have laid lots of tile, but only interior. Planning on using ditra over the newly poured concrete, versabond for the thinset, and tile tbd (probably some porcelain that looks like stone / rock with enough texture to mitigate slipping).

Am I missing something / any suggestions are greatly appreciated on either the concrete pour or the tile plan.
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Old 05-31-2019, 12:08 PM   #2
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Welcome back, Matthew.

1. You have removed the offending trees? How large will this new addition be?

2. You want any such patio sloped a minimum of 2% away from the structure (yeah, even in southern California) and my personal minimum is 2" below the threshold of any doors into the structure.

3. The minimum thickness of a poured concrete patio should be 4" and properly reinforced per local soil requirements.

4. With the use of Ditra, you can, per the Ditra manufacturer, install the Ditra and tile the day after the pour if you want.

My opinion; worth price charged.
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Old 05-31-2019, 01:01 PM   #3
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Thanks as always for the response CX. I know I speak for many when I say that you have no idea how much you have helped me over the years with several projects / questions both with responses to my questions as well as reading other posts where you have responded.

I actually spent a month last summer sitting on a mini-excavator removing not just the offending tree, but thirteen other 60-80 foot high sycamore trees from my front yard. Not sure why anyone thought planting that many trees that height that close together was going to work, but it has been on the to-do list since we moved in.

The patio size is staying the same so I just removed the bricks from the slab (see attached pictures).

I am planning on a standard slope (1/4" / foot) when I set the forms.

On the last two points, I searched high and low to find guidance on the thickness and understand the need for at least 4" on a slab. My thinking on this project is that I have an existing 6" slab so I am just putting 2" on top of it, but I could not find much guidance on this. Lots of discussion on things like micro-toppings, self-leveling compounds, etc, but not a lot on thickness of a standard pour on top of an existing slab.

As far as timing, my concern on tiling too soon after the pour is that the slab will shrink will curing and so waiting 28 days would mitigate that risk. I have tiled over recently installed Ditra before, but the underlying slab was not new.

Thanks again for all the input.
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Old 05-31-2019, 02:45 PM   #4
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I did not understand that you intended to put your new concrete over your existing, Matthew. Thought you were adding more area to your patio.

The customary minimum thickness for such pours is 2 inches. Doesn't look to me as though you could achieve that while keeping it under your door thresholds and providing slope. What am I missing this time?
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Old 05-31-2019, 03:50 PM   #5
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I was not explicit, but I am about to demo the last row of bricks along the house so did not want any confusion. Once that has been done I will have a slab that is approximately 2 1/4" below the door.

The bricks that have been removed are 2 1/4" so I have that much to play with at the door. To have a proper slope I would have less than 2" to play towards to front of the patio due to it being raised.

The options I considered are:

1 - pour 1 1/2" - 2" thick concrete over existing slab that has been prepped with a bonding agent and rebar that has been epoxied into holes that are drilled and spaced appropriately. Downside is this is a cold joint and also that that the pour would be too thin in some spaces. Upside is I do not need to jack any additional concrete.

2- self-leveling compound over existing slab. Downside is that I have used SLC only on smaller areas that need to be leveled vs. an entire patio so not familiar with this approach. Upside is no additional concrete demo.

3 - jack existing concrete so I can pour thicker than option #1. Downside is having to demo additional concrete.

Open to other options.
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Old 06-01-2019, 04:59 AM   #6
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Are there any slab cracks with elevation differences?

If the slab doesn’t have any cracks with height differences, I’d be more inclined to chip out the brick mortar bed to get down to the raw slab then place a bonded mud bed using deck mud before I’d consider pouring concrete over the existing. Deck mud will enable you to shape a flat and sloped surface perfect for tile or Ditra then tile if you prefer.
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Old 06-01-2019, 09:35 AM   #7
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Had not considered deck mud. The slab has no cracks so might be a good option.

Questions regarding this option:

1- would you suggest tying the deck mud into the slab via epoxied rebar or does it just sit atop the slab?

2 - is there any min or max thickness for deck mud? I need between 1" - 2" of height (tbd when I finalize the tile and know its thickness).

3 - should I be using something like wire mesh to reinforce the deck mud if it is at least 1" thick throughout?

4- should I use a bonding adhesive or a fortifier for the deck mud mix?

Thanks for the help.
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Old 06-01-2019, 10:51 AM   #8
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Unless someone disagrees, my updated plan of action is as follows:

1 - power wash existing slab to get all of the dust / residue from the brick demo.

2- lay deck mud 1- 2" thick with wire mesh placed appropriately to level slab with the sliding door frame allowing for enough room that the thinset and tile will be level with the bottom of the door frame.

3 - install ditra over deck mud using versabond (I think schluter calls for unmodified if ditra goes over concrete, but I have always used versabond so think this works?)

4 - install tile over ditra using versabond.

5 - grout - do I need to use a grout that is specific to exterior tile?
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Old 06-01-2019, 02:52 PM   #9
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Like Chris, I'd use deck mud to build it up. I'd use Versabond to bond the deck mud to the old concrete. Ditra will work but I'd probably use Redgard or Aqua Defense on top of the mud. This patio had pitch originally but they wanted to turn it into a sun room. So, I had 1 to 1 1/4 thick mud at the house and about 3 inches along the outside edge.
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Old 06-03-2019, 01:16 PM   #10
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Thanks for the reply. Sorry for the newbie question, but have not done a mud bed before and assumed it would just go over the concrete. In doing a little search I see lots of comments regarding bonded or unbonded mud bed, but some seem to reference using some type of reinforcement like wire mesh in the context of bonding.

Your picture shows the thinset troweled under the mud bed. To bond the mud bed do you just trowel the thinset and cover with the mud as you go along?
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Old 06-03-2019, 03:15 PM   #11
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Davy - that’s a great picture! Always make me want to shake your helper/mud maker’s hand!
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Old 06-03-2019, 07:21 PM   #12
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Thanks PC. That would be my son, he makes good mud.

Yes, on a slab, you can bond the mud with thinset like I show it. I make sure the thinset is wet and not dried out when I dump the mud on it. For an unbonded mud bed, you need 1 1/2 or thicker mud bed usually with a cleavage membrane such as felt paper and then welded wire within the mud. This patio wasn't thick enough near the house for an unbonded mud bed although I very seldom go unbonded.
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Old 06-04-2019, 10:00 AM   #13
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Thanks again to Davy, CX, and Chris. Went to set the forms yesterday and the front part of the patio is raised high enough that it is almost at the same height I need to start the bed against the house. Everything I can find indicates the mortar bed needs at least 1/2" (maybe 3/4") of thickness. I might be able to get 1/2", but it would be close so wondering if using something like Quikrete Sand Topping might be a better option as I have used that in the past when I need to get a feather edge.

I am going to add an acrylic fortifier due to how thin I need to get at the edge. I would use something like a SLC or resurfacer if I did not need to build the patio up 1" - 1 1/2" at the base of the house, but that would be expensive.

I keep telling my wife that this project is actually straight forward once i can get a level substrate to put the tile and promise her it will go quickly once that happens. My dispensation to get this done is quickly evaporating so will be finalizing forms today and hopefully mixing and pouring. In fairness, my history of estimating a time frame on a project like this is not good so her complaints are valid.

Will post pictures when I am done. Thanks again for the suggestions / help.
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Old 06-04-2019, 10:30 AM   #14
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Sand topping is too rich, for deck mud you want a 5:1 sand to cement. With a bonded bed I have no problem going 1/2” against the house I wouldn’t add any acrylic that’ll make it hard to cut. With deck mud you don’t need forms, take your time with setting up some perimeter screeds like Davy’s picture and go to town filling in between.
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Old 06-04-2019, 10:36 AM   #15
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I also looked at that quantity of mud wondering if a mixer or slave labor. That son must be buff.
What are negatives with to rich mixrure such as dand topping like i used in my better bench? It did get wet on top as i worked the final grade.
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